Spinal manipulative therapy is an independent risk factor for vertebral artery dissection

To the Editor:

We read with interest the article by Smith et al.1 The authors state that spinal manipulative therapy (SMT) is strongly and independently associated with dissection of the vertebral artery leading to stroke or transient ischemic attack (TIA). These results are consistent with the only other epidemiologic study on the topic.2 One strength of this study is the attempt to control for confounding by indication as measured by neck pain before the stroke. However, several methodologic issues that threaten the validity of case-control studies require clarification. The first two relate to selection bias and the third one to information bias.

First, the selection of controls may explain some of the reported association between SMT and vertebral artery dissection (VAD). By selecting controls with stroke other than dissection, the authors have selected a control series that is sicker that the case series. Our main concern is that the controls were sampled from a population that did not give rise to the cases. This is evidenced (authors’ table 2) by the higher proportion of significant comorbidities among the control group. Several studies have reported that patients with comorbidities or poorer health status are less likely to seek chiropractic care.3-5⇓⇓ Therefore, by design, the authors may have selected a control series that was less likely to receive SMT. This would artificially inflate the association between dissection leading to stroke/TIA and SMT.

Second, a history of SMT to the cervical spine may have influenced the diagnosis …